BRAF positive Langerhans cell sarcoma arising from CALR positive myeloproliferative neoplasm: evidence of a clonal progenitor.

Langerhans cell sarcoma (LCS) is an aggressive malignant neoplasm with a Langerhans cell immunophenotype and high-grade cytological features. Occasionally, it can coexist with other hematopoietic neoplasms with proven clonal relationship. Most of these neoplasms were found to be of lymphoid origin. This phenomenon is usually explained as lineage plasticity. Although, to the best of our knowledge, the coexistence of LCS and myeloproliferative neoplasm (MPN) with proven clonal relationship has never been reported. Herein, we describe a case of a 79-year-old man with MPN, who developed LCS 2 years after original diagnosis. Biopsy and pathological evaluation revealed a high-grade malignant neoplasm, identified as LCS with expressing CD1a, langerin, and S100. The next-generation sequencing performed on skin and bone marrow biopsies showed identical mutations in ASXL1, CALR, and TET2 genes, while the LCS tumor showed an additional BRAF mutation. This case presents the case of progression of MPN to LCS supported by molecular evidence.